The present invention relates to two-component plastic systems useful for surgically filling voids in bones. More specifically, the invention relates to polymer cements comprising a liquid component and a dry component wherein the dry component includes polymer beads.
Polymer based surgical cement systems have been successfully employed for many years to fill voids in bones. Such cements have found their greatest use in the fixation of orthopaedic implants. Typically a bone is cut to accommodate an implant and then liquid and dry components of the cement system are mixed to form a paste which is applied to the cut bone. The implant is seated in this paste, which, when fully polymerized, forms a continuous solid interface between the implant and bone. The invention of this disclosure encompasses improvements in these polymer bone cements. To better understand the invention, it will be helpful to review the basic composition, behavior, and deficiencies of prior cements.
FIG. 1 depicts a typical prior art dry component of a bone cement system. The dry component includes a loose mixture of polymer beads 1, polymer flakes (or milled beads) 2, and a powdered opacifier 3. The beads contain a polymerization initiator such as benzoyl peroxide (BPO). Typically these beads are formed in a solution polymerization process in which BPO is added as a polymerization initiator to a monomer and polymerization is carried out. BPO added in excess of that required for polymerization of the monomer remains in the polymer as a residual. The more BPO added, the greater will be the residual BPO randomly distributed in the polymerized beads, within practical limits. However, the molecular weight of the resulting beads decreases as the BPO is increased. A high molecular weight is important in bone cement beads because mechanical strength increases as molecular weight increases. The tradeoff between residual BPO and molecular weight has limited the residual BPO attainable in beads having a useful molecular weight. For example, it is very difficult to produce a bead with a molecular weight greater than 500,000 and a residual BPO content greater than 2% by weight. When the residual BPO content is below approximately 1% by weight, the addition of free BPO powder to the mixture comprising the dry cement component may be required to achieve a desired set time, typically between 10 and 15 minutes. However, uniform dispersion of this BPO powder is difficult. Finally, the opacifier 3 is included to color the cement to aid its implantation and to make it visible on a radiograph. The opacifier tends to form clumps 4 because it is a fine powder added to the beads and flakes.
In use the dry component is mixed with the liquid component which contains a monomer and typically an amine accelerator such as N,N-Dimethyl-p-toluidine (DMPT). Upon mixing, the monomer dissolves the flake polymer to a great extent due to the large surface area of the flake, thereby creating a viscous fluid or paste. In addition, the monomer begins to dissolve the beads at a much slower rate than the flake because of the relatively small surface area of the beads. As the beads partially dissolve, residual BPO becomes available to the monomer. The BPO decomposes in the presence of DMPT into free radicals which act as polymerization initiators for the monomer, and polymer chains begin forming from the beads outwardly. However, only the BPO that is exposed by bead dissolution is accessible, and the beads only partially dissolve. Since the BPO is distributed throughout the bead, the usable BPO concentration of prior art cements is less than the actual concentration in the bead. As polymerization progresses, the cement paste grows more viscous until it eventually hardens into a solid. It is helpful to characterized this hardening process as having three stages. FIG. 2 depicts a viscosity versus time graph for a typical polymer bone cement- During the first, or mixing stage, the cement components are mixed and a viscous paste, represented by .mu..sub.1, is formed primarily due to the dissolution of the polymer flake in the monomer. During the second stage, or working time, the paste is of a suitable viscosity to be effectively applied to the bone. By design this may be a fairly thick putty-like consistency suitable for manually packing into the bone or it may be a thinner flowable consistency suitable for injection into the bone. The consistency can by controlled, for example, by varying the ratio of flake to beads in the dry component. It is noted that absent the BPO this stage would continue for a considerable period with only slight thickening due to further dissolution of the beads. However, because of the BPO, polymerization takes place and the paste reaches a state, represented by .mu..sub.2, where it is no longer able to be worked. The polymerization reaction, which is exothermic, continues during the final stage until the cement is fully hardened. The entire process typically takes from two to fifteen minutes. Because of the practical limits on the amount of residual BPO and its distribution throughout the beads, the viscosity versus time curve, as shown in FIG. 2, for prior art cements is not readily tailored.
The resulting solid cement, as depicted in FIG. 3, comprises a matrix of polymerized plastic 5 containing a distribution of beads 1 and opacifier 3. The beads are generally firmly attached to the matrix since the polymer chains formed from the beads outwardly as BPO was exposed. However, if free BPO was added to raise BPO levels, then when the dry cement component having a non-uniform dispersion of BPO powder is mixed with the liquid cement component, polymerization will proceed more quickly at regions of relatively high BPO concentration. These regions will be outside of the beads, resulting in localized and less uniform polymerization which can result in reduced mechanical properties. Furthermore, the opacifier is simply encased in the matrix and forms no attachment with it, thereby concentrating stresses placed on the cement and weakening it. The tendency of the opacifier to clump 4 can further weaken the cement and the clumps can obscure the radiographic image of the cement-bone interface. U.S. Pat. Nos. 4,791,150 to Braden et al. and 4,500,658 to Fox describe cements having an opacifier dispersed in polymer cement beads during the bead formation. The references teach the use of a suspension polymerization batch process for forming beads as discussed above and further teach including the opacifier particles in the suspension polymerization solution so that the beads formed will contain some opacifier. This method of incorporating opacifier is tedious and costly to use. It also produces a bead with an uncontrolled and random opacifier distribution. As a result some of the opacifier particles will likely by located near the bead surface, allowing the particles to become exposed and separated from the bead when the surface is dissolved by the monomer during use. Such separated particles will be deposited in the matrix and can form stress concentrators as previously described.